[IDB IGNORE] [MDB IGNORE] HUD datum cleanup.

[MistakeNot4892]

Description of changes

Collapses HUD icons and icon updates onto /datum/hud where possible.
Adds a helper (/mob/proc/get_target_zone()) to replace using the zone selector screen element directly.
Oh god it spiralled out of control.

TODO:

• Uncomment/implement the periodic HUD icon updates using the commented code.
• Split the construct icons up into several sets.
• Update existing hud types to use the correct elements.
• Work out the ordering issue with alerts/element updates.
• Go back over Life() and check if reordering the parent calls has cooked anything.
• Test that everything is placed properly and functions as expected.

Why and what will this PR improve

The current HUD code duplicates a ton of logic and element creation. This should make it much easier to handle HUDs in general.

Authorship

Myself.

Changelog

Ideally, nothing player facing.

[MistakeNot4892]

Closing as stale, big chunks have been picked out to other PRs anyway.